Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity in next generation Wireless Local Area Network (WLAN) systems. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques. A multiple antenna communication system increases the data rate in a given channel bandwidth by transmitting separate data streams on multiple transmit antennas. Each receiver receives a combination of these data streams on multiple receive antennas.
In order to properly receive the different data streams, receivers in a multiple antenna communication system must acquire the channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation. It is desirable for multiple antenna communication systems to co-exist with legacy single antenna communications systems (typically referred to as Single Input Single Output (SISO) systems). Thus, a legacy (single antenna) communications system must be able to interpret the preambles that are transmitted by multiple antenna communication systems. Most legacy Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). Generally, the preamble signal seen by the legacy device should allow for accurate synchronization and channel estimation for the part of the packet that the legacy device needs to understand. Previous MIMO preamble formats have reused the legacy training preamble to reduce the overhead and improve efficiency. Generally, the proposed MIMO preamble formats include the legacy training preamble and additional long training symbols, such that the extended MIMO preamble format includes at least one long training symbol for each transmit antenna or spatial stream.
A number of frame formats have been proposed for evolving multiple antenna communication systems, such as MIMO-OFDM systems. Existing frame formats provide inaccurate estimations for the MIMO systems, such as inaccurate power measurement or outdated frequency offset and timing offset information, or fail to provide full backwards compatibility to the legacy devices of some vendors. In one proposed MIMO frame format associated with the 802.11n standard, each transmit antenna sequentially transmits one or more long training symbols (LTS), such that only one transmit antenna is active at a time. Such a per-antenna training scheme requires sufficient transmit antenna isolation in the PHY architecture for MIMO channel estimation during the long training sequence. Thus, while the active antenna is transmitting, the remaining transmit antennas must be “silent” for the receiver to properly obtain the channel coefficients from the received signals. Proper isolation of one antenna and its transmitter chain to another is critical to avoid excessive RF leakage onto the “silent” transmitters, resulting in corrupted channel estimation from the desired transmitter.
In one prior isolation technique, the “silent” transmit antenna chains (typically comprising a digital signal processor, RF transceiver and power amplifier) were switched on and off. Such switching of the antenna chains, however, will cause the temperature of the corresponding power amplifier to increase and decrease, respectively. Generally, such heating and cooling of the power amplifier will lead to “breathing” effects that cause the transmitted signal to have a phase or magnitude offset, relative to the desired signal. In addition, turning off the antenna chain may also cause glitches in the voltage controlled oscillator (VCO) in the RF transceiver as well as excessive delays due to the start-up time of the power amplifiers.
A need therefore exists for methods and systems for performing channel estimation and training in a MIMO-OFDM system with improved antenna isolation.